Spatial analysis of ship-strike risk for Rice’s whale in the Gulf of Mexico

Benjamin D Best, PhD (ben@ecoquants.com)

2024-02-23

Introduction

Motivation. The Rice’s whale (Balaenoptera ricei) is endangered with an estimated population size of 51 individuals (Garrison et al. 2020). The Biological Opinion (NOAA 2020) that proposes vessel mitigation measures is based on a distribution model (Roberts et al. 2016) that has been superseded by a newer model (Litz et al. 2022) showing range expansion into the western Gulf of Mexico.

Methods. We adhered to similar methods as in the Biological Opinion (NOAA 2020) for creating a new Whale Area also based on the 100- to 400- meter isobaths to now extend to the western Gulf of Mexico, except without a 10 km buffer (since only marginal improvement from 94% to 97% containment of distribution). Risk to ship strike was based on the same AIS vessel traffic data (2014-2018).

Whale Distributions

Whales Captured

Table 1:

Table of new whale densities (Litz et al. 2022) summarized by total study area (U.S. Gulf of Mexico), previous Whale Area (NMFS 2020) and newly proposed Whale Area.
Item # %
Whales in Study (U.S. Gulf of Mexico) 51 100%
Whales in Original Whale Area (NMFS, 2020) 27 52%
Whales in New Whale Area 48 94%

Vessel Traffic

Figure 1: Map of annual average traffic (km) for all vessel types at all speeds from AIS data (2014 to 2018). Depth contours are shown in dash blacked lines for 100 m (finer) and 400 m (thicker).
Figure 2: Map of annual average traffic (km) for oil and gas vessels at all speeds from AIS data (2014 to 2018). Depth contours are shown in dash blacked lines for 100 m (finer) and 400 m (thicker).
Figure 3: Map of annual average traffic (km) for all vessel types > 10 knots from AIS data (2014 to 2018). Depth contours are shown in dash blacked lines for 100 m (finer) and 400 m (thicker).
Figure 4: Map of annual average traffic (km) for oil and gas vessels > 10 knots from AIS data (2014 to 2018). Depth contours are shown in dash blacked lines for 100 m (finer) and 400 m (thicker).

Vessel Risk to Whales

\[ Risk = {Whale\:density}\:(\#/100km^2) * {Vessel\:traffic\:(km/100km^2)} \]

Figure 5: Map of risk (# whales * km vessel traffic) for all vessels at all speeds. Depth contours are shown in dash blacked lines for 100 m (finer) and 400 m (thicker).
Figure 6: Map of risk (# whales * km vessel traffic) for oil and gas vessels > 10 knots. Depth contours are shown in dash blacked lines for 100 m (finer) and 400 m (thicker).

Risk Reduction by Areas

Table 2:

Reduction of vessel strike risk (# whales * km vessel traffic) to Rice’s whales with enforcement of original (NMFS 2020) and new Whale Areas, averaged across years of AIS data (2014 to 2018). All percentage (%) reductions are compared to All Vessels for given speeds.
Speed - Vessel Type Risk Reduction by Whale Area
Original % New %
All speeds - All vessels 11,072 12% 84,667 93%
All speeds - Oil & Gas vessels 2,010 2% 32,911 36%
> 10 knots - All vessels 8,196 11% 66,952 93%
> 10 knots - Oil & Gas vessels 695 1% 21,001 29%

Thank You

This report received funding from Earthjustice and NRDC. The AIS data used in (NMFS 2020) were provided by NMFS pursuant to a Freedom of Information Act request. A portion of the background section was contributed by Jay Barlow. Steve Mashuda (Earthjustice) and Michael Jasny (NRDC) reviewed drafts of the report.

Further Information